The present invention relates to a rectifier for a DC/DC converter and, more particularly, to a synchronous rectifier using a MOSFET instead of a rectifying diode.
A conventional MOSFET rectifier for a DC/DC converter constituting a forward converter is disclosed in "Effect of Gate Resistance on the Efficiency of the MOSFET Synchronous Rectifier" in B-532 of Proceedings of the 1991 IEICE Fall Conference, (September, 1991). As shown in FIG. 5, in this rectifier, a rectifying n-channel MOSFET (to be referred to as a rectifying FET hereinafter) 4 is connected in series with a secondary winding 2b of a main transformer 2, and the series circuit constituted by the secondary winding 2i b of the main transformer 2 and the rectifying FET 4 is connected in parallel with a commutating n-channel MOSFET (to be referred to as a commutating FET hereinafter) 9. The source of the rectifying FET 4 is connected to the source of the commutating FET 9. The gate of the rectifying FET 4 is connected to the drain of the commutating FET 9 through a gate resistor 16. The gate of the commutating FET 9 is connected to the drain of the rectifying FET 4 through a gate resistor 17. A filter circuit constituted by a choke coil 14 and a capacitor 15 is connected to the output of the converter. Referring to FIG. 5, reference numeral 1 denotes an input filter circuit connected to the input of a primary winding 2a of the main transformer 2; 3, a main switch connected in series with the primary winding 2a of the main transformer 2; 5, a parasitic diode of the rectifying FET 4; 6, an input capacitor of the rectifying FET 4; 10, a parasitic diode of the commutating FET 9; and 11, an input capacitor of the commutating FET 9.
An operation of this conventional circuit will be described next. When the main switch 3 is turned on, and a switching current flows in the primary winding 2a of the main transformer 2 through the input filter circuit 1, a voltage is induced in the secondary winding 2b of the main transformer 2. As a result, a charge current of the input capacitor 6 flows through a loop constituted by the secondary winding 2b of the main transformer 2, the gate resistor 16, the input capacitor 6 of the rectifying FET 4, the parasitic diode 5 of the rectifying FET 4, and the secondary winding 2b of the main transformer 2. With this operation, when the gate potential of the rectifying FET 4 increases, with respect to the source potential (secondary side GND), to a threshold potential or more in a period of time determined by the time constant of the resistor 16 and the input capacitor 6, the rectifying FET 4 is turned on.
When the rectifying FET 4 is turned on, a discharge current of the input capacitor 11 flows through a loop constituted by the input capacitor 11, the gate resistor 17, the rectifying FET 4, and the input capacitor 11. When the gate potential of the commutating FET 9 decreases, with respect to the source potential (secondary side GND), to a threshold potential or less in a period of time determined by the time constant of the gate resistor 17 and the input capacitor 11, the commutating FET 9 is turned off. At this time, a load current flows through a loop constituted by the secondary winding 2b of the main transformer 2, the choke coil 14, the load (not shown) of the converter, the rectifying FET 4, and the secondary winding 2b of the main transformer 2.
When the main switch 3 is turned off, a flyback voltage is generated in the secondary winding 2b of the main transformer 2 as the switching current flowing in the primary winding 2a of the main transformer 2 is cut off. Consequently, with a reverse operation to that performed when the main switch 3 is turned on, a charge current flows in the input capacitor 11 through the parasitic diode 10 to turn on the commutating FET 9. A discharge current from the input capacitor 6 then flows via the rectifying FET 4 to turn off the rectifying FET 4. As a result, a load current flows through a loop constituted by the choke coil 14, the load (not shown) of the converter, the commutating FET 9, and the choke coil 14.
In this conventional MOSFET rectifier for a DC/DC converter, however, the operations of the rectifying FET 4 and the commutating FET 9 are delayed with respect to the operation of the main switch 3 owing to the time constants of the gate resistors 16 and 17 and the input capacitors 6 and 11 of the FETs 4 and 9. As the ON timings of the rectifying FET 4 and the commutating FET 9 upon switching of the main switch 3 are delayed, the recovery loss due to an increase in the ON time of each of the parasitic diodes 5 and 10 of the FETs 4 and 9 is increased. On the other hand, as the OFF timings of the rectifying FET 4 and the commutating FET 9 upon switching of the main switch 3 are delayed, the short-circuit time of the secondary winding 2b of the main transformer 2 is prolonged, resulting in a great reduction in efficiency.